feat(settings): quick wins from the settings audit (#453)

- #456 Copy pass: sentence case for every setting title and section,
  plainer names ("Hide joins and leaves", "Link previews", "Emoji
  style: Twemoji", "Rejoin calls after a restart", "Also use the
  browser's noise suppression", "Push-to-talk key"...) and grammar fixes
  ("when messages arrive", "Play a sound when a new message arrives",
  blocked users "Enter their Matrix ID"). Labels only: no setting is
  renamed in storage.
- #455 One theme control: Theme: System / Light / Dark. System shows the
  light and dark variant pickers, Light or Dark one picker of that kind.
  Replaces the System switch plus a Theme dropdown that was greyed out
  whenever the switch was on. Uses the existing settings (no migration);
  picking a variant also remembers it for System.
- #457 Remove the "Block Messages" stub from Notifications; the GIF
  picker row only appears in builds with a GIF key.
- #458 (part 1) "Export/Import Messages Data" -> "Export/Import
  encryption keys", with a sentence on when you need it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/settings/developer/CryptoDiagnostics.tsx

@@ -35,7 +35,7 @@ export function CryptoDiagnostics() {
 
   return (
     <Box direction="Column" gap="100">
-      <Text size="L400">Crypto Diagnostics</Text>
+      <Text size="L400">Crypto diagnostics</Text>
       <SequenceCard
         className={SequenceCardStyle}
         variant="SurfaceVariant"
@@ -43,8 +43,8 @@ export function CryptoDiagnostics() {
         gap="400"
       >
         <SettingTile
-          title="Crypto Diagnostics — captures E2EE error signatures this session"
-          description="Ring-buffers up to 200 matched console warnings/errors for the KE-1..KE-4 bug cluster. Local only — no network calls. The downloaded report includes the matched log lines as evidence."
+          title="Crypto diagnostics"
+          description="Captures encryption error signatures from this session. Ring-buffers up to 200 matched console warnings/errors for the KE-1..KE-4 bug cluster. Local only — no network calls. The downloaded report includes the matched log lines as evidence."
           after={
             <Box alignItems="Center" gap="200" shrink="No">
               <Badge variant={count > 0 ? 'Critical' : 'Secondary'} fill="Solid" radii="Pill">